Hundreds of thousands of people across Europe will today commemorate the 20th anniversary of the world's worst human-made disaster - when Reactor 4 at the Chernobyl nuclear power complex exploded during a routine safety test and sent a plume of radioactivity a mile high to drift over 40% of Europe and as far away as Japan. Between 50 and 250 million curies of radiation, approximately equal to 100 medium-sized atomic bombs, was unleashed.

But there will be no celebrations of progress in Ukraine - home to Chernobyl - Russia, or Belarus, the three most affected countries, which said yesterday they were struggling with a legacy of ill-health, poverty and psychological illnesses affecting their people.

Yesterday Belarus said that one-sixth of the country was still contaminated and the disaster had cost it $235bn (£131.5bn) so far. In a separate study, scientists said the health of the 200,000 people in Ukraine who took part in the cleanup had been badly affected. Across the region, hospitals said they were overwhelmed by people with thyroid cancers, children with genetic mutations and adolescents with radiation-linked illnesses.

But while five independent scientific studies in the last two weeks, including one by the Russia's academy of sciences, have estimated that between 30,000 and 250,000 people have died so far as a result of the disaster, yesterday the World Health Organisation maintained its figure that only 50 people died and that it expects perhaps 9,000 to die eventually from the accident.

Greg Hartl, a WHO spokesman, said a generation of people had become deeply disturbed and poverty and lifestyle illnesses had scarred large populations. "The relocation of people proved a ... traumatic experience because of disruption to social networks and the impossibility of returning home," he said.

Although vast areas across the three countries are technically uninhabitable, several hundred people have either refused to go or are now moving in to the abandoned houses in the "dead zone" surrounding the reactor.

In Chernobyl, Ivan Benidenko, his wife Natalya and father Petro have moved back, but say they get their food supplies from outside. "I am from here, I like living here. Now we are trying to rebuild a life," said Ivan. "This is the birth of a new community. It is our big hope, even though there are no children."

On the edge of a forest, Maria Urupa and her husband Mikhail were both victims who refused to leave in 1986 and now take their chances. They eat contaminated berries and mushrooms. "I am afraid of nothing here. Our neighbours were very ill, and many who left have died, and my son had stomach illnesses for years but we are lucky. We would die if we left," she said.

Meanwhile, the future of the reactor is uncertain. A sarcophagus of nearly 700,000 tonnes of steel and 400,000 tonnes of concrete was hastily built to seal the reactor in 1986, but this is now leaking and close to the end of its life. Vince Novak, the director of the European Bank for Reconstruction and Development's nuclear safety department who is in charge of the bank's Chernobyl project said: "How the Soviets built [the sarcophagus] in 1986 for me is almost a miracle. I can't think of anyone else being able to do it in the way they did it."

The plan now is to construct a giant steel arch, costing nearly $1.2bn, which will be installed over the site. But this will not be ready for five years.

The structure will last 100 years but because 200 tonnes of uranium and almost a tonne of elements including plutonium are still inside the power station, as well as vast amounts of radioactive water and dust, the reactor may have to be monitored forever.